Boiler Pressure Too High – Find Out Why and How to Reduce it

Boilers are meant to heat your home and supply hot water to your taps. For a consistent performance, the boiler pressure must remain within a certain limit. A low boiler pressure may deprive you of heat and hot water, whereas in the event of a high boiler pressure, the problem might not be an interruption in heating; rather, it may result in water leakage or a complete boiler breakdown. A consistently high pressure may end up seriously damaging the entire heating system. 

A slight variation in the boiler pressure is not something to worry about; it’s just normal. When the boiler is cold, the pressure often drops, whereas it rises when it is hot. But if the pressure enters the danger zone, often marked red on the pressure meter, it’s time to take action. The immediate solution to this problem is to release the pressure so that it drops down and comes within the safe limits. 

Understanding Safe Limits of Boiler Pressure

Boiler pressure plays a key role in the optimum performance of a boiler. The majority of the boilers in the UK are combi or system boilers. These are designed to perform safely and efficiently as per the following pressure limits:

• If the Boiler Pressure is below 1 bar, it falls in the category of low pressure. The boiler will not perform as expected at this pressure.
• If the boiler is cold, it would perform perfectly at a pressure ranging between 1 to 1.5 bar.
• If the boiler is hot, the pressure may rise up to 1.8 to 2.5 bar. 
• If the boiler pressure exceeds 2.5 bar, whether cold or hot, it is termed “too high”.
• If the boiler pressure exceeds 3 bar (danger zone), it is a sign that the system is over-pressurized and needs immediate attention. 

The above parameters clearly indicate that the safe limits of a boiler pressure range between 1 to 1.5 bar when the boiler is in a cold state and up to 2.5 bar if the boiler is hot. Anything less than 1 and beyond 2.5 bar is abnormal, whereas a pressure more than 3 bar is an indication that the boiler has entered into danger zone. 

At a boiler pressure (in hot state) exceeding 2.5 bar, water may start discharging from the pressure relief valve. This is a clear sign that the boiler pressure is too high. If that happens, you need to take immediate action to reduce the pressure, to protect it from entering the danger zone, i.e., a pressure exceeding 3 bar. Under normal circumstances, high pressure can be reduced through bleeding radiators. However, if the pressure remains consistently above 3 bar, despite bleeding radiators, it may be time to call a qualified boiler engineer to resolve the issue. 

Remember, normally a high boiler pressure may not affect heating and hot water supply, but it may badly affect the entire heating system, whereas the possibility of bursting of the pipework is also there.

Reasons for High Boiler Pressure

There might be several reasons behind the high boiler pressure. Often, one or more of the following situations, combined together, may cause a high boiler pressure: 

• Overfilling

Overfilling of the system is one of the most common reasons for high boiler pressure. It happens when you top up the pressure through a filling loop. It often happens that after topping up the pressure, the filling valve is not tightened or left open. In this scenario, water continues to enter the system, resulting in the development of high pressure. 

• Fault or Error in Expansion Vessel

An expansion vessel is an important internal component of a boiler, which absorbs water expansion during the process of heating it. At times, a fault may occur in its diaphragm, which results in an abrupt increase in water pressure. Another possibility is that the pressure vessel may lose its air charge. Both situations can result in sudden and consistent high pressure that indicates a fault in the expansion vessel. 

• Pressure Relief Valve Not Working

A faulty Pressure Relief Valve (PRV) is another very common reason behind high boiler pressure. PRV is meant to release excess pressure when the pressure level enters the red zone. Sometimes, the PRV might be jammed or malfunction due to other reasons. In both cases, it fails to release excess pressure, leading to an increase in boiler pressure to a potentially dangerous level. 

• Limescale or Sludge

In some areas, the water supplied comes under the category of “hard water”. The boiler, where hard water is supplied, tends to build up limescale or sludge. It results in blockages, which may also cause variation in the boiler pressure. In these areas, high boiler pressure is often a result of limescale buildup in the internal components of the boiler.

• Overfilling Instead of Fixing Leaks

A prevalent reason for low boiler pressure is leaks in the central heating system. Sometimes, this situation is handled by repeatedly topping up the pressure, without fixing the leaks. Although this action resolves the low boiler pressure instantly but the actual problem remains unattended. If you consistently top up the pressure without addressing the underlying issue of water leaks, it may lead to excessive water intake, ultimately causing high boiler pressure. So, the practice of topping up pressure without fixing leaks is a wrong practice, which should be avoided. 

• Repairing Old Parts or Fitting New Parts

Servicing, maintenance, and repairing of old parts or fitting new parts in the system can often result in trapping of air within the system. This situation may also lead to high boiler pressure.    

Impact of High Boiler Pressure on the System

An unattended and continuous high boiler pressure is highly dangerous for the heating system as well as the boiler itself. Some of the most common impacts of high boiler pressure on the system are as follows:

• The immediate impact of high boiler pressure is that the pipes and radiators start leaking. In most severe cases, the boiler seals may also be damaged, resulting in leaks. In most severe situations, some of the pipes or radiators may also burst. 
• The heat exchanger and the expansion vessel are quite sensitive components of the boiler. A consistent high pressure may potentially damage these important components. 
• Most of the modern boilers are often fitted with automatic safety shutdown devices. In the event of unattended high boiler pressure, this device is automatically activated, resulting in boiler lockout. 
• High boiler pressure significantly impacts energy efficiency, leading to abnormally high energy bills. 
• Often, the warranties are subject to timely servicing and maintenance of the boilers. A consistent state of high boiler pressure can affect the warranty, as it needs to be fixed timely. If the problem is not fixed, the warranty may be invalid.

How to Reduce Boiler Pressure

Given the compelling impact of high boiler pressure, you must manage it immediately. In most cases, the pressure can be reduced easily, whereas in more severe and consistent cases, you may need to call for the assistance of a qualified engineer. 

Here is a step-by-step process of reducing the boiler pressure:

• First of all, you need to turn off the Boiler and let it cool down before any further action.
• Once the boiler is sufficiently cooled down, search for the filling loop. Ideally, you should be well aware of the location of such components. 
• Once located, check that both of the valves attached to the filling loop are tightly closed. If not closed properly, do it now. A slight opening of any of these valves may result in excess intake of water, causing high boiler pressure. 
• Next, you may proceed to bleed the radiators. This can be done with the help of a radiator key. While doing so, it is recommended to place a small container or cloth underneath the valve. Slowly start opening the valve. Wait till water starts coming out from the valve. Let the water drain out. Side by side, keep monitoring the pressure meter. When the pressure drops to around 1.3 bar (in cold state), stop bleeding the radiator. Tightly close the valve to stop water from draining out further.
• This should ideally serve the purpose. However, sometimes it may not work as expected. In this scenario, you may need to drain out some water from the system, using the drain-off valve, often located at the bottom of the heating system.
• Keep draining the water while monitoring the gauge. Once the desired pressure is achieved, stop further draining. 
• This is now the time to restart your boiler. Keep an eye on the pressure meter. 
• Under normal conditions, this should resolve the problem of high pressure in your boiler. However, if it again rises beyond 3 bars, don’t try anymore; it’s time to call a boiler engineer to check and fix the actual issue thoroughly.

IMPORTANT

• Check the boiler pressure at least once a month.
• Never top up fast; do it slowly.
• Ensure annual servicing and inspection.
• Fix even the small leaks instantly. 

Call a Gas Safe Registered Engineer

In the following situations, never delay calling a Gas Safe Registered Engineer to resolve the issue:

• If the boiler pressure keeps rising even after bleeding radiators.
• If you are in doubt about the efficiency of the expansion vessel.
• If your boiler produces gurgling or banging sounds.
• If you notice a visible leak. 

Conclusion

Water pressure in a boiler plays a vital role in its optimum performance. The normal pressure ranges from 1 to 1.5 bar when the boiler is cold. If the boiler is hot, the pressure may rise to 2.5. If the pressure drops even below 1 bar, you may face interruption in the hot water supply and non-heating of your rooms. However, it does not pose any threat to you or your heating system. On the other hand, if the pressure exceeds 2.5 bars, the danger starts. 

Anything above 3 bars is an alarming situation that needs to be resolved without delay. Otherwise, the boiler and heating system may be seriously damaged, including leaks and internal component damage. Under normal conditions, high boiler pressure can be easily managed by bleeding the radiators. However, in more severe faults, you may need to call a Gas Safe-registered engineer for a complete inspection and to fix the problem.